The present invention generally relates to radiation emitter assemblies such as, for example, light emitting diode (LED) packages and to heat dissipating packages for electronic components.
Radiation emitters, particularly optical radiation emitters, are used in a wide variety of commercial and industrial products and systems, and accordingly come in many forms and packages. As used herein, the term “optical radiation emitter” includes all emitter devices that emit visible light, near infrared (IR) radiation, and ultraviolet (UV) radiation. Such optical radiation emitters may be photoluminescent, electroluminescent, or other solid state emitter. Photoluminescent sources include phosphorescent and fluorescent sources. Fluorescent sources include phosphors and fluorescent dyes, pigments, crystals, substrates, coatings, and other materials.
Electroluminescent sources include semiconductor optical radiation emitters and other devices that emit optical radiation in response to electrical excitation. Semiconductor optical radiation emitters include light emitting diode (LED) chips, light emitting polymers (LEPs), organic light emitting devices (OLEDs), polymer light emitting devices (PLEDs), etc.
Semiconductor optical emitter components, particularly LED devices, have become commonplace in a wide variety of consumer and industrial opto-electronic applications. Other types of semiconductor optical emitter components, including OLEDs, LEPs, and the like, may also be packaged in discrete components suitable as substitutes for conventional inorganic LEDs in many of these applications.
Visible LED components of all colors are used alone or in small clusters as status indicators on such products as computer monitors, coffee makers, stereo receivers, CD players, VCRs, and the like. Such indicators are also found in a diversity of systems such as instrument panels in aircraft, trains, ships, cars, trucks, minivans and sport utility vehicles, etc. Addressable arrays containing hundreds or thousands of visible LED components are found in moving-message displays such as those found in many airports and stock market trading centers and also as high brightness large-area outdoor television screens found in many sports complexes and on some urban billboards.
Amber, red, and red-orange emitting visible LEDs are used in arrays of up to 100 components in visual signaling systems such as vehicle center high-mounted stop lamps (CHMSLs), brake lamps, exterior turn signals and hazard flashers, exterior signaling mirrors, and for roadway construction hazard markers. Amber, red, and blue-green emitting visible LEDs are increasingly being used in much larger arrays of up to 300 components as stop/slow/go lights at intersections in urban and suburban intersections.
Multi-color combinations of pluralities of visible colored LEDs are being used as the source of projected white light for illumination in binary-complementary and ternary RGB illuminators. Such illuminators are useful as vehicle or aircraft maplights, for example, or as vehicle or aircraft reading or courtesy lights, cargo lights, license plate illuminators, backup lights, and exterior mirror puddle lights. Other pertinent uses include portable flashlights and other illuminator applications where rugged, compact, lightweight, high efficiency, long-life, low voltage sources of white illumination are needed. Phosphor-enhanced “white” LEDs may also be used in some of these instances as illuminators.
IR emitting LEDs are being used for remote control and communication in such devices as VCR, TV, CD, and other audio-visual remote control units. Similarly, high intensity IR-emitting LEDs are being used for communication between IRDA devices such as desktop, laptop and palmtop computers; PDAs (personal digital assistants); and computer peripherals such as printers, network adapters, pointing devices (“mice,” trackballs, etc.), keyboards, and other computers. IR LED emitters and IR receivers also serve as sensors for proximity or presence in industrial control systems, for location or orientation within such opto-electronic devices such as pointing devices and optical encoders, and as read heads in such systems as barcode scanners. IR LED emitters may also be used in a night vision system for automobiles.
Blue, violet, and UV emitting LEDs and LED lasers are being used extensively for data storage and retrieval applications such as reading and writing to high-density optical storage disks.
Performance and reliability of LED components, chips, and systems are heavily influenced by the thermal performance of those components, chips, and systems, and by ambient temperature. Elevated operating temperatures simultaneously reduce the emission efficiency of LEDs and increase the probability of failure in most conditions. This elevated temperature may be the result of high system thermal resistance acting in concert with internal LED power dissipation and may also be the result of high ambient operating temperature or other influence. Regardless of the cause, LED efficiency and reliability are normal adversely affected by increases in temperature. Thus, it is advantageous to minimize temperature rise of LED components, chips, and systems attributable to internal power dissipation during operation. This can be accomplished by reducing the conductive, convective, and radiative thermal resistance between the LED chip and ambient environment, such as by optimizing the materials and construction of the packaged device containing the LED chip. These methods, as applicable to mass-solderable, auto-insertable, and other discrete LED components, are disclosed in commonly assigned U.S. Pat. No. 6,335,548, entitled “SEMICONDUCTOR RADIATION EMITTER PACKAGE,” filed on Oct. 22, 1999, by John K. Roberts et al., and published PCT International Publication No. WO 00/55914.
For high power LED systems and high power density LED systems, system thermal performance is especially critical. LED illuminators and high power signal lights generating more than ten lumens (or more than one watt of power dissipation) are examples of systems which can benefit from improved thermal performance, especially if package area/volume must be minimized (increasing power density).
To limit the operational temperature of the LED, the power that is allowed to be dissipated through the LED is typically limited. To limit the dissipated power, however, the current that may be passed through the LED must be limited, which in turn limits the emitted flux of the LED since the emitted flux is typically proportional to the electrical current passed through the LED.
Other fundamental properties of LEDs place further restrictions on the useful operational temperature change ΔT. Semiconductor LEDs, including IR, visible, and UV emitters, emit light via the physical mechanism of electro-luminescence. Their emission is characteristic of the band gap of the materials from which they are composed and their quantum efficiency varies inversely with their internal temperature. An increase in LED chip temperature results in a corresponding decrease in their emission efficiency. This effect is quite significant for all common types of LEDs for visible, UV, and IR emission. Commonly, a 1° C. increase (ΔT) in chip temperature typically results in up to a 1 percent reduction in useful radiation and up to a 0.1 nm shift in the peak wavelength of the emission, assuming operation at a constant power. Thus, a ΔT of 40° C. can result in up to a 40 percent reduction in emitted flux and a 4 nm shift in peak wavelength.
From the preceding discussion, it can be seen that to avoid thermal damage and achieve optimal LED emission performance, it is very important to minimize the ΔT experienced by the LED device chip and package during operation. This may be achieved by limiting power or reducing thermal resistance.
Limiting LED power, of course, is antithetical to the purpose of high power LEDs, i.e., to produce more useful radiation. Generating higher flux with an LED generally requires higher current (and therefore higher power). Most prior art devices, however, exhibit relatively high thermal resistance from their semiconductor radiation emitter to ambient and are compelled to limit power dissipation in order to avoid internal damage. Thus, the best 5 mm T-1¾ THD packages are limited to about 110 mW continuous power dissipation at 25° C. ambient temperature.
An additional problem faced by designers of conventional LED devices is that the wire bond used to join one of the LED leads to the LED chip can break or lose contact with the lead or the chip. Such failure can occur, for example, due to shear forces that are transferred to the wire bond through the encapsulant or thermal expansion/contraction of the encapsulant around the wire bond.
The other forms of radiation emitters mentioned above also experience performance degradation, damage, increased failure probability or accelerated decay if exposed to excessive operating temperatures.
Consequently, it is desirable to provide a radiation emitter device that has a higher emission output than conventional LED devices while being less susceptible to failure due to a break in the wire bond contact or other defect that may be caused by excessive operating temperatures.
Similar heat dissipation problems exist with respect to other electronic components. For example, large heat sinks are often attached to microprocessors of the type used in personal computers. Accordingly, an improved heat dissipation package for such electronic components is desirable.